Tamm-Horsfall glycoprotein binds IgG with high affinity

Tamm-Horsfall protein (THP), a monomeric glycoprotein (M(r) 80 to 100 kDa), is produced by the mammalian kidney's thick ascending limb of Henle cells and excreted into the urine. The function of THP is uncertain. Here we report that a high molecular weight contaminant in sheep THP (sTHP) preparations was identified as sheep IgG by its positive reaction with donkey anti-sheep IgG antibody and with protein G. To answer the question of whether sTHP and sheep IgG co-purified because of a physical interaction between the two proteins, an enzyme-linked immunosorbent assay (ELISA) using immobilized sTHP and soluble sheep IgG was performed. Analysis of the ELISA data identified the presence of two sets of binding sites: a high affinity site (Kd 10(-8) to 10(-9) M) and a lower affinity site (Kd 10(-6) to 10(-7) M) [corrected]. The ELISA detected a similar high affinity interaction between human THP (hTHP) and human IgG. The binding of sheep IgG to immobilized sTHP was inhibited by soluble sTHP. These observations suggest an additional factor to be considered in studies addressing THP's potential immunoregulatory function.